Method of and apparatus for induction heating of small areas



METHOD '0 AND APPARATUS FOR INDUCTION HEATING OF SMALL AREAS Filed April 24, 1946 Jan. 10, 1950 o M. MARQUARDT Er AL 2,493,773

510.4: mllmm INVENTORS. ttoMMarquardf WdlcenqMcy' es ka.

5 ATTORNEY Patented Jan. 10, 1950 :6?

METHOD OF AND APPARATUS FOR INDUC- TION HEATING OF SMALL AREAS Otto M. Marquardt, Oakland Township, Oakland County, and WalterA. Majeska, Grosse Pointe, Mich, assignors, by mesne assignments, to The Ohio Crankshaft Company, Cleveland, Ohio, a

corporation of Ohio Application April 24, 1946, Serial No. 664,478

3 Claims. (01.219 13) This invention relates to inductionheating, with particular application to the heating ofsmall flat areas.

According to recognized methods of inductively heating small areas such as rod ends, a coil of any suitable electric conductor is constructed with a central axial opening made bythe turns of the coil, and in this opening the bar, the end of which is to be heated, is inserted. On the application of electrical power to the coil heat arising from the alternating magnetic flux permeates the rod end and heats the same to a desired temperature whereupon the rod is quenched by appropriate means and a hardened end is secured which adapts the rod for tappet or other uses.

A deficiency arising from this method of bar heating is that by insertion of the rod end in the coil too large a mass of the rod is heated. In a tappet rod, for example, it is necessary only that the extreme end he heat treated. Within the opening of the coil it is impossible to avoid a substantial amount of heating for a relatively large distance from the rod end even though a single-turn coil is used.

It is an object of the present invention to provide a construction for an induction heating" coil which permits heat treatment of a metal surface in a closely restricted region.

Another object of the invention is to provide a heating method which will insure substantial uniformity of heating across the area heated.

A related object is the securing of a uniform heating in a small area for a limited depth only.

An object also is to provide an inductive heating coil with means for securing at will any desired heat pattern across the face of a workpiece adjoining the heating coil of the heating apparatus.

Referring to the drawings: Figure 1 is a plan view .of the induction coil with the insulation plate broken away to show the core;

Figure 2 is a section along lines 2-2 of Figure 1 showing the heat pattern on the workpiece;

Figure 3 is a view of a coil provided with a circular core;

Figure 4 is a section along lines 4--4 of Figure 3 and showing also the heat pattern on a Workpiece;

Figure 5 is a view of the conventional heating means for a rod end; and

Figure 6 is a sectional view along lines 6-6 of Figure 5 showing the heat pattern on the rod.

Prior art methods of developing high tempera- I tures in small areas, such as a tappet rod end,

have followed the procedure shown in Figures 5 and 6 in which a single-turn induction coil I formed of the parallel terminal ends 2 and 3 and the circular enclosing coil turn 4 is adapted to re,-v ceive a cylindrical rod end 5 for heating. The passage of current between the terminals 2 and 3 of the coil develops therein magnetic flux fol-- lowing the general pattern indicated by dashed lines 6 in Figure 6, these lines penetrating the permeable ierro-magnetic rod end and through.

hysteresis and eddy current developing a temperature therein exceeding the critical temperature for magnetic effects. The heat pattern is shown in Figure 6, and it is apparent that the depth of heating is substantial and in excess of that required for end heating, such as would be necessary in a tappet rod. As a result the energy absorbed by the unit is greatly in excess of that necessary; and wastage results. The applicants in considering this problem of wastage and the desirability of limiting the heating effect to the zone immediately adjacent the end of the rod, succeeded in securing improved resultsby the arrangement shown in Figures 3 and 4. In the construction of these figures a U-shaped heating turn [0 is employed enclosing a heating chamber II in which a cylindrical lining l2 ofinsulation is placed enclosing a plu rality of ferro-magnetic wire strands l3, such as iron. These strands are closely packed together and coated with insulating material so as to reduce or eliminate interstrand electrical con-' ductivity. Appropriate work plates of dielectric substance may cover the substance of the coil and-the ends of the core to support the workpiece, but in the particular form as shown this plate has been omitted. It is apparent from the heat pattern of Figure 4 that the magnetic flux pene-.

trates the edge of the rod 29 with greater density and with a resultant higher heating eifect so.

that after the heat treatment, by quenching, the hardened areas are at the edge of the end of the rod. This, while not afiording a uniformly hardened region, is useful in such cases where edge hardness is paramount in need.

The form of disclosure of Figures 1 and 2 has been found to be particularly useful for securing a heat pattern of substantially uniform and reduced thickness. In this form of the construction a U-shaped coil turn I5 is employed having parallel terminal sections [6 and I! with appropriate apertures I8 for fastening to electrical conductors. The inner bend or bight of the coil is shaped as a Greek cross and lined with insu-.

3 lation, as at l9, and within the insulation a plurality of ferro-magnetic wire strands 20, Such as iron, each insulated, is closely packed to form the core 2|. As shown in Figure 2, the ends of this core are flush 'with the *plane of *the coil sides, and overlying the ends and the adjoining section of the coil are dielectric plates 22 and 23. These plates serve to form a support for the end of the rod or any other workpiece which is to be heated and to maintain the wire strands in place within the coil turn.

The purpose of using a cmiciform core is to cause a distribution of the -inagnetic flux with a greater concentration toward thebriterdf the core than in the region approachingthe'circum"-' ference of the circle with a diameter equal to the maximum cross-sectional dimension'of thecore. In other words, a cylinder through the core having a diameter equal to the minimum diameter or cross-sectional dimension of the core from points 24 to 25 will have greater average flux concentration perunit-area than the region between' this cylinder and a cylinderwhose diameter eXtendsbetWeenpQintS 2'6 and i1. Consequentlyjther'ewill be atendency for the'magnetic flux to extend into-the rod tip 30 a greaterdistance toward the center than would occur if this variation in the coreden'sity-did not exist. The heat pattern of Figure 4 indicates the result where there is a uniform'distribution of'the core material throughout the large diameter thereof. Accordingly, "by varying the average density of the core structure "from axis "to major circumference; i. 'e., in any circumferential zone, the flux pattern ma be 'niodifiedan'd the heat .pattern consequently changed as desired either-to secure a uniform heating or variations from uniformity to edgeheatingy'as shownin l igure :2.

For illustrative purposes certain structure'has been specified which is not necessary'for the objects of the invention. "For'example, insulation wire strands have been indicated as the core substance. Obviously other core materials may be used, such aspowdered iron or solid magnetic metal. Also, a; cruciform core-has been indicated as desirable. To securemodified heatingeffects the core may be star shaped'orfollow any other polygonal shape, as desired. Other variationsof the invention may-be'madeand hence no'limit'a 60 tion is implied by the specific disclosure other than that may be required 'bytheclaimsashereto appended.

What-is claimed is:

-1. A method'of rapidly-heating an area-of a' generally flat metal surface with a high frequency magnetic fiux of a frequencysuch that it does not substantially penetrate below the sur face-of the metal comprising the steps of :gene'rati'ng an alternating magnetic fl'ux fii'eld entirely on one side of said "surface andpf such a freing a coil including-one or more turns of a con- -ductive material adapted to have a high-fre- "q'u'ency alternating current circulated therethrough, said coil defining a central opening and having a work side at one side of the opening adapted to-be positioned in proximity to the surface to be heated, said opening having a magnetic core disposed therein with both ends of said core positioned on one side of a plane passing through theplan'e of'said surface, said core having anavera'ge magnetic density for any complete circumferential zone which continuously decreases at least from 'a point spaced from its central axis to the outer edge thereof.

"3. Electrical induction -heating apparatus adaptedto heat metal surfaces of a lesser area than the overall "area of 'the apparatus comprisi'n'g a'coil'includin'g oneormore turns of an electrically conductive member defining a central opening, said coil adapted to be positioned in proximitytothe surfaceto behea'ted'with one end or -said "opening facingsaidsurface, a magnetic core member in said opening, said core member having a "central portionan'd 'aplur'ality of projections extending generally radially from said portion, 's'aidprojectio'ns forming a core of decreasing 'average ma'gnetic de'nsity'in any complete circumferential zone outwardly from said portion.

OTTO M. MARQUARDT. WALTER A. 'MAJESKA.

REFERENCES CITED The following references are of record in the file of this .patent:

UNITED STATES PATENTS Number Name Date 1,052,119 Anderson Feb. 4, 19'13 2,182,341 Hulster Dec. 5, 1939 2 ,352,332 Limpel June 27, 1944 2,378,890 Abe et a1 June 26, 1945 FOREIGN PATENTS Number Country Date 883 859 Great "Britain NOV. 24, 1932 444,003 Great Britain Mar. 11, '1936 655,097 France Apr. 15, 1929 #159434 France Aug. 25, 1931 

